Electrochemical capacitors (ECs), also known as ultracapacitors or supercapacitors, are being considered for uses in hybrid electric vehicles (EVs) where they can supplement a battery used in an electric car to provide bursts of power needed for rapid acceleration. A battery would still be used for cruising, but supercapacitors (with their ability to release energy much more quickly than batteries) would kick in whenever the car needs to accelerate for merging, passing, emergency maneuvers, and hill climbing. The supercapacitor must also store sufficient energy to provide an acceptable driving range. To be cost- and weight-effective compared to additional battery capacity they must combine adequate specific energy and specific power with long cycle life, and meet cost targets as well. Specifically, it must store about 400 Wh of energy, be able to deliver about 40 kW of power for about 10 seconds, and provide high cycle-life (>100,000 cycles).
The high volumetric capacitance density of a supercapacitor (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective “plate area” and from storing energy in the diffuse double layer. This electric double layer (EDL), created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only about 1 nm, thus forming an extremely small effective “plate separation.” In some supercapacitors, stored energy is further augmented by pseudo-capacitance effects, occurring again at the solid-electrolyte interface due to electrochemical phenomena such as the redox charge transfer. The double layer capacitor is based on a high surface area electrode material, such as activated carbon, immersed in an electrolyte. A polarized double layer is formed at electrode-electrolyte interfaces providing high capacitance. This implies that the specific capacitance of a supercapacitor is directly proportional to the specific surface area of the electrode material. This surface area must be accessible by electrolyte and the resulting interfacial zones must be sufficiently large to accommodate the so-called double-layer charges.
Due to the notion that the formation of EDLs does not involve a chemical reaction, such as certain reduction-oxidation (redox) reactions, the charge-discharge process of a supercapacitor can be very fast, typically in seconds, resulting in very high power density. Supercapacitors are extremely attractive power sources. Compared with batteries, they require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, experience no “memory effect,” and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life. Perhaps most importantly, supercapacitors offer higher power density than batteries.
Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for vehicle power or renewable energy storage applications. For instance, supercapacitors possess very high power densities, but low energy densities when compared to batteries (5-8 Wh/kg for commercial supercapacitors vs. 10-30 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery).
On the other hand, lithium-ion batteries possess a much higher energy density (100-180 Wh/kg), but deliver a very low power density (100-500 W/Kg), requiring typically hours for re-charge. Lithium-ion batteries also pose some safety concern. The low power density or long re-charge time of a lithium ion battery is due to the mechanism with which lithium ions are shuttled between a negative electrode (anode) and a positive electrode (cathode), in which lithium ions must enter or intercalate into, for instance, inter-planar spaces of a graphite crystal at the anode (during re-charge) and into the complex oxide crystal (e.g. lithium cobalt oxide or lithium iron titanate) or other lithium insertion compound at the cathode. This intercalation or diffusion process requires a long time to accomplish. For instance, the current lithium-ion battery for plug-in hybrid vehicles requires 2-6 hours of recharge time, as opposed to just seconds for supercapacitors.
Table 1 provides a comparison of characteristics between conventional supercapacitors and conventional lithium-ion batteries.
TABLE 1Comparison between supercapacitors and lithium-ion batteries.Ultracapacitors orLithium-Ion CharacteristicsSupercapacitorsBatteriesMain AdvantagesHigh power density; Moderate power density;Long cycle life;High energy densityRecharged in seconds; Relatively safeMainLow energy densitySafety concern; Disadvantageslong recharge timeEnergy Density3-6 Wh/kg (conventional)100-180 Wh/kgPower Density5,000-10,000 W/Kg100-500 W/KgDeep Cycle Life500,000-1,000,000500-2000Cycling Efficiency >95%<80%Cell Voltage2.7 V3.6/3.7 VTemperature Range−50° C.-50° C.−10° C.-50° C.Discharge TimeSecondsMinutes to hourRecharge TimeSecondsHours
The above discussion suggests that an energy storage device that is capable of storing as much energy as a battery and yet can be fully recharged in one or two minutes like a supercapacitor would be considered a revolutionary advancement in energy technology. This was precisely the objective of our research and development efforts that led to the instant invention.
Instead of using inorganic lithium insertion compound, such as LiCoO2 and LiFePO4, that require lithium insertion and extraction from a bulk inorganic particle (typically 100 nm-20 μm, but more typically 1-10 μm), several attempts have been made to use organic molecules or polymers as an electrode active material for the cathode (lithium metal as the anode). For instance, Le Gall, et at investigated Poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene) as an organic polymer cathode [T. Le Gall, et al. J. Power Sources, 119 (2003) 316-320] and Chen, et at used LixC6O6 organic electrode, obtained from a renewable source, in a lithium ion battery [H. Chen, et al. “From biomass to a renewable LixC6O6 organic electrode for sustainable Li-ion batteries,” ChemSusChem, 1 (2008) 348-355]. Unfortunately, these organic materials exhibit very poor electronic conductivity and, hence, electrons could not be quickly collected or could not be collected at all. Although these organic molecules contain carbonyl groups (>C═O) that presumably could readily react with lithium ions (forming a redox pair), this redox mechanism was overwhelmed by the poor electronic conductivity. As a result, the battery cells featuring these organic molecules exhibit poor power densities. Le Gall et at added a large proportion of conductive acetylene black (typically 40-60% by weight) to partially overcome the conductivity issue; but, acetylene black significantly dilutes the amount of the active material. Further, the best achievable specific capacity of 150 mAh/g is far less than the theoretical specific capacity of 705 mAh/g of Poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene).
Recently, more electrically conducting carbon nano-tubes (CNTs) containing carbonyl groups were used by Lee, et at to replace the organic molecules for use as a cathode material [S. W. Lee, et al, “High Power Lithium Batteries from Functionalized Carbon Nanotubes,” Nature Nanotechnology, 5 (2010) 531-537]. The significantly higher electronic conductivity of CNTs does serve to overcome the poor conductivity problem of organic molecules. However, the CNT-based approach still suffers from several relatively insurmountable technical and economical issues, which call into question the commercial viability or utility value of this approach. Some of these issues are:                (1) CNTs are known to be extremely expensive due to the low yield, low production rate, and low purification rate commonly associated with the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs.        (2) CNTs tend to form a tangled mess resembling a hairball, which is difficult to work with (e.g., difficult to disperse in a liquid solvent or resin matrix).        (3) The so-called “layer-by-layer” approach (LBL) used by Lee, et al is a slow and expensive process that is not amenable to large-scale fabrication of battery electrodes. The LBL process has not been used in any significant amount of commercialized products.        (4) The LBL provides quality electrodes of up to 3 or 4 μm in thickness only. A useful battery or supercapacitor electrode thickness is typically in the range of 50-500 μm.        (5) CNT-based electrodes prepared without using the LBL approach did not show particularly good performance. There was no data to prove that CNT-based electrodes of practical thickness could even work due to the poor dispersion and electrolyte inaccessibility issues.        (6) CNTs have very limited amount of suitable sites to accept any functional group without damaging the basal plane or graphene plane structure. A CNT only has one end that is readily functionalizable and this end is an extremely small proportion of the total CNT surface. By chemically functionalizing the exterior basal plane, one would dramatically compromise the electronic conductivity of a CNT.        
Hence, there exists an urgent need to develop a new class of highly conducting electrode materials having a functional group that is capable of forming a redox reaction with lithium ions. These materials must have an adequate amount of readily functionalizable sites to host a desired amount of useful functional groups. These materials must be mass-producible with low costs. These materials must be stable in a wide temperature range (e.g. −40° C. to 60° C., a commonly operating range for a battery). After an extensive and intensive research and development work, we have discovered a new type of electrode materials that meet this set of stringent technical and economical requirements.
This new type of materials includes chemically functionalized exfoliated graphite (or graphite worms composed of inter-connected graphite flakes) and the chemically functionalized nano graphene platelets (f-NGPs) that are isolated and separated graphite flakes containing a layer of graphene plane or a plurality of layers of graphene planes with a thickness less than 100 nm. These NGPs can be obtained by severing the interconnections between flakes in a graphite worm. In other words, an NGP is an individual basal plane of carbon atoms (a single-layer graphene sheet) or a stack of multiple graphene sheets. A single-layer graphene sheet is basically a 2-D hexagon lattice of sp2 carbon atoms covalently bonded along two plane directions. The sheet is essentially one carbon atom thick, which is smaller than 0.34 nm. In the presently invented lithium super-battery, the interconnected graphite flakes in a graphite worm and/or the separated/isolated NGPs have certain specific functional groups capable of reversibly and rapidly forming a redox pair with a lithium ion during the charge and discharge cycles of a battery cell.
Both exfoliated graphite and NGPs are obtained from a precursor material, such as graphite particles, using a low-cost process. As one example of the production processes, natural graphite powder may be immersed in a mixture of sulfuric acid, nitric acid, and potassium permanganate at room temperature for 2-96 hours. The resulting material is a graphite intercalation compound (GIC) or graphite oxide (GO). This material is then subjected to a thermal shock (e.g., 1,000° C. for 30-60 minutes) to obtain worm-like graphite structure commonly referred to as exfoliated graphite or graphite worms. A graphite worm is composed of many loosely interconnected graphite flakes with multiple pores that are of 1 nm to several microns in size. This is a weak and fluffy, 3-D material. Graphite worms may then be subjected to mechanical shearing (e.g., milling) or ultrasonication treatment to produce nano graphene platelets (NGPs) that are isolated, separated graphite structures typically composed of single graphene plane or multiple graphene planes. By definition, an NGP is an isolated, separated platelet having a thickness less than 100 nm. However, the NGPs in the instant invention are more typically less than 10 nm in thickness. In most cases, they contain 1-20 layers of graphene planes.
In the present context, NGPs refer to pristine, non-oxidized graphene, graphene oxide (GO), or chemically or thermally reduced GO. The oxygen content is typically ranged from essentially 0% (pristine graphene) to approximately 45% (heavily oxidized graphite or graphene oxide). The chemically or thermally reduced GO typically has an oxygen content from 1% to 25%, more typically from 1% to 5%. When oxidized, a single-layer graphene sheet has a thickness in the range of perhaps 0.5-1.0 nm.
Nano graphene materials have recently been found to exhibit exceptionally high thermal conductivity, high electrical conductivity, and high strength. As a matter of fact, single-layer graphene exhibits the highest thermal conductivity and highest intrinsic strength of all currently known materials. Another outstanding characteristic of graphene is its exceptionally high specific surface area. A single graphene sheet provides a specific external surface area of approximately 2,675 m2/g (that is accessible by liquid electrolyte), as opposed to the exterior surface area of approximately 1,300 m2/g provided by a corresponding single-wall CNT (interior surface not accessible by electrolyte). The electrical conductivity of graphene is slightly higher than that of CNTs.
Two of the instant applicants (A. Zhamu and B. Z. Jang) and their colleagues were the first to investigate NGP- and other nano graphite-based nano materials for supercapacitor application [L. Song, A. Zhamu, J. Guo, and B. Z. Jang “Nano-scaled Graphene Plate Nanocomposites for Supercapacitor Electrodes” U.S. Pat. No. 7,623,340 (Nov. 24, 2009); application submitted in 2006]. After 2007, researchers began to gradually realize the significance of nano graphene materials for supercapacitor applications [e.g., M. D. Stoller, et al, “Graphene-based Ultracapacitor,” Nano Letters, Vo. 8 (2008) pp. 3498-3502]. However, these supercapacitors are not a lithium ion battery in which lithium ions are shuttled between an anode and a cathode.
In other prior investigations, non-functionalized NGPs were used as either (A) an anode active material (wherein the inter-planar spaces in a multiple-layer NGP serve as a host for intercalated lithium atoms) or (B) a supporting material for an anode active material (e.g., Si particles or coating adhered to the graphene surface and it is Si that absorbs or desorbs lithium) or cathode active material (e.g., LiCoO2 and LiFePO4, which are also lithium intercalation compounds). In each and every one of these earlier studies, lithium ions or atoms are intercalated or inserted into the interior crystal structure of a lithium intercalation compound and this insertion or extraction procedure is slow. None of these devices rely on select functional groups (attached at the edge or basal plane surfaces of a graphene sheet or platelet) that readily form a redox pair with a lithium ion from a lithium-containing electrolyte. Due to this slow process of lithium diffusion in and out of these intercalation compounds, these conventional lithium ion batteries do not exhibit a high power density and the batteries require a long re-charge time.
In contrast, the presently invented lithium super-battery relies on fast, reversible formation of a redox pair between a graphene-borne functional group and a lithium ion in the electrolyte. Since no intercalation involved, this process is fast and can occurs in seconds or even shorter. Hence, this is a totally new class of battery that exhibits an unparalleled and unprecedented combined performance of an exceptional power density, high energy density, long and stable cycle life, and wide operating temperature range.